The present invention relates to a gray scale expression method for use in a display device and, particularly, to a gray scale expression method adequate to suppress pseudo contours of moving images in displaying gray scale on a flat type display device such as plasma display panel and a gray scale display device using the same method.
In general, a plasma display panel (referred to as "PDP", hereinafter) has many merits such as thin structure, free from flicker, large display contrast ratio, possibility of providing a relatively large screen, high response speed and possibility of multi-color emission by utilizing fluorescent material of self emission type, etc., and, recently, its use in such fields as display devices related to computer and color image display is becoming popular.
The PDP can be classified, according to an operation system thereof, to an AC discharge type in which electrodes are coated with dielectric material and are operated in an indirect AC discharging state and a DC discharge type in which electrodes are exposed in a discharge space and operated in a direct discharge state. The AC discharge type PDP is further classified, according to a drive system, to a memory operation type which utilizes a discharge cell memory and a refresh operation type which does not utilize such memory. Incidentally, light intensity of the PDP is substantially proportional to a discharge frequency, that is, a repetition frequency of pulse voltage. Since light intensity of the refresh type PDP is lowered when its display capacity becomes large, the refresh type PDP is mainly used for small display capacity.
FIG. 14 is a cross section of an example of the A.C. discharge memory operation type PDP, showing a construction of a display cell schematically. The display cell a rear insulating substrate 1 and a front insulating substrate 2, both of which are of glass, a transparent scan electrode 3 formed on an inner surface of the front insulating substrate 2, a transparent sustaining electrode 4 also formed on the inner surface of the front insulating substrate 2, trace electrodes 5 and 6 formed on surfaces of the scan electrode 3 and the sustaining electrode 4 in order to reduce electrode resistances, respectively, a data electrode 7 formed on an inner surface of the rear insulating substrate 1 perpendicularly to the scan electrode 3 and the sustaining electrode 4, a discharge gas space 8 provided between the insulating substrates 1 and 2 and filled with a discharge gas such as helium, neon or xenon or a mixture of them, partition walls 9 for maintaining the discharge gas space 8 and partitioning between display cells, a fluorescent material 11 for converting ultra-violet ray generated by a discharge of the discharge gas in the space 8 into a visible light 10, a dielectric member 12 covering the scan electrode 3 and the sustaining electrode 4, a protective layer 13 formed of magnesium oxide, etc., for protecting the dielectric member 12 against discharge and a dielectric member 14 covering the data electrode 7.
A discharge operation of a selected display cell will be described with reference to FIG. 14. When a discharge is started by applying a pulse voltage exceeding a discharge threshold value across the scan electrode 3 and the data electrode 4, positive and negative electric charges are attracted to the respective dielectric members 12 and 14 and accumulated thereon correspondingly to the polarity of this pulse voltage. Since an internal voltage equivalent to the accumulated charge, that is, the wall voltage, has a polarity opposite to the polarity of the pulse voltage, an effective voltage within the cell is lowered with growth of discharge and it becomes impossible to sustain the discharge even when the pulse voltage is kept constant. Thus, the discharge is ultimately stopped. Thereafter, when a sustaining pulse which is a pulse voltage having the same polarity as that of the wall voltage is applied across the scan electrode 3 and the sustaining electrode 4, it is possible to discharge even if the voltage amplitude of the sustaining pulse is small, since the wall voltage is added to the sustaining pulse voltage as an effective voltage, resulting in a drive voltage exceeding the discharge threshold value.
Therefore, it becomes possible to maintain discharge by continuously applying the sustaining pulse across the scan electrode 3 and the sustaining electrode 4. This function is the above mentioned memory function. Further, it is possible to stop the sustaining discharge by applying a low voltage pulse having large width or an erase pulse having a small width similar to the sustaining pulse voltage across the scan electrode 3 and the sustaining electrode 4 such that the wall voltage is neutralized.
FIG. 15 shows conventional drive waveforms such as disclosed in SOCIETY FOR INFORMATION DISPLAY INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS VOLUME XXVI, pp807, for driving a plasma display panel having a structure such as shown in FIG. 16.
The panel shown in FIG. 16 is for a dot matrix display panel including j (column electrodes).times.k (line electrodes). That is, the panel includes scan electrodes Sc1, Sc2, . . . , Scj and sustaining electrodes Su1, Su2, . . . , Suj arranged in parallel to the respective scan electrodes, as the column electrodes and data electrodes D1, D2, . . . , Dk arranged perpendicularly to each of the column electrodes, as the line electrodes.
In FIG. 15, a sustaining electrode drive waveform Wu applied commonly to the sustaining electrodes Su1, Su2, . . . , Suj, scan electrode drive waveforms Ws1, Ws2, . . . , Wsj applied to the respective scan electrodes Sc1, Sc2, . . . , Scj and a data electrode drive waveform Wd applied to the data electrode Di are shown, where 1.ltoreq.i.ltoreq.k. A drive period includes a preliminary discharge period A, a write discharge period B and a sustaining discharge period C and a desired image display is obtained by repeating the drive period.
The preliminary discharge period A includes a preliminary discharge pulse Pp for discharging all of the display cells of the PDP panel 15 and preliminary discharge erase pulses Pp.sub.e for extinguishing charges among the wall charges produced by the application of the preliminary discharge pulse, which impedes the write discharge and the sustaining discharge. In the preliminary discharge period A, active particles and the wall charges which are necessary to obtain a stable write discharge characteristics in the write discharge period B are produced in the discharge gas space.
In the sustaining discharge period C, in order to obtain desired light intensity of the display cells which are subjected to the write discharge in the write discharge period B, the discharges of the display cells are sustained.
In the preliminary discharge period A, the preliminary discharge pulse Pp is supplied to the sustaining electrodes Su1, Su2, . . . , Suj to discharge all of the display cells. Then, the erase pulses Pp.sub.e are applied to the scan electrodes Sc1, Sc2, . . . , Scj to produce erase discharges therein to thereby erase the wall charges accumulated by the preliminary discharge pulse.
Thereafter, in the write period B, the scan pulse Pw is applied to the scan electrodes Sc1, Sc2, . . . , Scj in line-sequence and the data pulse Pd is selectively applied to the data electrodes Di correspondingly to video display data, to produce discharges in the display cells to be displayed to thereby produce the wall charges.
Finally, in the sustaining discharge period C, the discharges of only the display cells in which the write discharges occur are sustained by the sustaining pulses Pc and Ps, completing a light emitting operation of the whole PDP panel.
A conventional sub-field display scheme for 64 gray levels, in which the scanning and sustaining drives are performed separately and which is utilized in an AC color plasma display, will be briefly described with reference to FIG. 17(a). One TV field which is usually in the order of one-sixtieth second (about 16.7 ms) at which flicker is negligible is divided into 6 sub-fields SF1.about.SF6 as shown in FIG. 17(a), each sub-field consisting of a scan period and a sustaining period.
In the scanning period of the sub-field SF1 of the sub-fields SF1.about.SF6, the write operation is performed for the respective pixels on the basis of display data of B5 which is the most significant bit number. After the write operation for the whole PDP panel completes, the sustaining discharge pulse is applied to the whole panel to emit light from only the written pixels. Then, the same drive is performed in the sub-field SF5, and so on. In order to obtain sufficient amount of light emission in the sustaining discharge periods of the respective sub-fields, the sustaining pulse is applied, for example, 256 times in the sub-field SF6, 128 times in the sub-field SF5, 64 times in the sub-field SF4, 32 times in the sub-field SF3, 16 times in the sub-field SF2 and 8 times in the sub-field SF1.
The above mentioned operation is basically the same as that shown in FIG. 17(b) which shows another conventional sub-field display scheme of a mixed scanning/sustaining drive type in which the write/erase scanning and the sustaining discharging are performed simultaneously or of a mixed drive type in which the scanning/sustaining are performed across adjacent sub-fields. Such sub-field scheme has to be employed due to the necessity of modulation of intensity of emitted light with the number of light emissions or the light emitting period and, in order to scan a plurality of times in each sub-field necessarily, the sub-field scheme requires a high speed scan and write operations within a short time. However, with the recent improvement of the write performance of the plasma display panel, a high speed write operation has become possible even at 3 microseconds or shorter and a full color display with 256 gray levels has been realized by using an 8 sub-field system.
Although such sub-field system is adequate to display still images, it has been found that disturbances of gradation are often observed when displaying moving images, dependent on image. For example, in a case where an image such as a human cheek having a slow spatial variation of gray levels moves on a display screen, pseudo contours which are darker or brighter or different in color from that of the cheek may appear on a portion of the cheek which is to be a smooth image. Further, there may also occur color separation or reduction of resolution. Such pseudo contours or gradation disturbances of moving images are very conspicuous in boarder regions of a smoothly varying gradation where gray levels jump up to higher bits, resulting in substantial degradation of display quality and image quality.
FIG. 18 shows a portion of gradation realized by combinations of 8 sub-fields SF1.about.SF8 weighted respectively by light intensities 128, 64, 32, 16, 8, 4, 2 and 1 corresponding to respective binary numbers each consisting 8 bits B7, B6, B5, B4, B3, B2, B1 and B0. By combining these sub-fields, it becomes possible to display 256 gray levels. That is, the light intensity of each of the 256 gray levels of each pixel can be realized by a binary number of 8 bits, B7.about.B0. Images are sequentially displayed by the sub-fields SF1.about.SF8 whose existence or absence of light intensities 128, 64, 32, 16, 8, 4, 2 and 1 is represented by binary numbers of the bits B7.about.B0, resulting in a natural image expressed by intermediate gray levels obtained by the integration effect of human eyes.
In FIG. 18, particularly, in a case where light intensity is varied by one gray level from 127 to 128, values of all of B6 to B0 are changed from "1" to "0" and a value of B7 is changed from "0" to "1". Therefore, when a PDP is activated in time from the lowest sub-field SF1 to the highest sub-field SF8 in the order, the light emitting period is substantially changed from a former half portion of a field to a later half thereof, resulting in the pseudo contours of moving images.
In order to solve this problem, a number of methods have been proposed. In Takigawa, "TV Display by AC Plasma Panel", the journal of Electronics & Communications Association of Japan, 77/Vol. J60-A, No. 1, pp. 56 to 62, it is described that it is effective to arrange sub-fields such that an average of light intensity within a time corresponding to one field becomes small at times preceding and succeeding to a shift-up or shift-down of bit and, in a case of display with 5 bits, that is, in 32 gray levels, a sub-field arrangement of SF3, SF2, SF1, SF5, SF4 with a light emitting period of higher bit being arranged in a center portion is effective to suppress pseudo contours of moving images. Further, it is also effective for the same purpose to reduce a display time within one field and, according to experiments conducted by him, a good display is realized by shortening the display period to one fourth of one field in the above sub-field arrangement.
Further, in A. Kohgami, "Gray Scale Display System of TV using Memory Type Gas Discharge Panel", Technical Report of Electronic Information Communications Association of Japan, EID90-9, 1990, it is described that pseudo contours of moving images can be improved by making a time interval from a first bit of a field to a last bit of a succeeding field within 20 milliseconds corresponding to a critical flicker frequency of human visual organ. Kohgami also describes that such time interval of 20 milliseconds or shorter can be realized by not arranging sub-fields throughout one field but arranging them dense in one side portion of the field similarly to the above mentioned Takigawa method.
Kohgami further describes that the above condition can also be satisfied by dividing and arranging high significant bits having long light emitting period. In a case of a 8-bit display, it is possible to realize the time of 18.8 milliseconds from the first bit of one field to a last bit of a next field by dividing the most significant bit B7 by 2 to obtain sub-fields SF8-1 and SF8-2, dividing a next significant bit B6 by 2 to obtain sub-field SF7-1 and SF7-2 and arranging the sub-fields SF8-1, SF8-2, SF7-1 and SF7-2 thus obtained discretely to constitute one field consisting of 10 sub-fields arranged in the order of SF7-1, SF8-1, SF1, SF2, SF3, SF4, SF5, SF6, SF7-2 and SF8-2, resulting in improved gray scale expression of moving images.
It should be noted that, in the present invention, the expression generally used in the field of the information processing is used such that the least significant bit, n-th significant bit and the lowest sub-field are expressed by B0, Bn-1 and SF1, respectively, although, in Kohgami, the most significant bit of a binary number representing the weight of light intensity is made Bl and the most significant sub-field corresponding thereto is made SF1.
There are other proposals for improvement on the contour disturbances of moving images by means of optimization of the arrangement of sub-fields. In Japanese Patent Application Laid-open No. H3-145691, a sub-field of a bit next to the most significant bit and a sub-field of a bit succeeding to the next bit are arranged on both sides of a sub-field of the most significant bit.
In Japanese Patent Application Laid-open No. H7-7702, a sub-field of the most significant bit is arranged in a center position and sub-fields of a next bit next to the most significant bit and a bit next to the next bit are arranged in opposite ends of a field which is separated in time from the sub-field of the most significant bit so as to disperse these sub-fields as far as possible.
Further, in Japanese Patent Application laid-open No. H7-271325, for 64 gray levels, pseudo contours of moving images, which occur when light intensity weighted with binary number is shifted up, is slightly suppressed by preparing three sub-fields (SF4-1, SF4-2, SF4-3) each of light intensity level of 8 and two sub-fields (SF5-1, SF5-2) each of light intensity level of 16 and, in displaying a light intensity in a range from light intensity level 16 to 23 and a range from light intensity level 48 to 55, producing gradation by switching between a first sub-field arrangement in which SF4-1 is selected and a second sub-field arrangement in which SF4-2 is selected, every scan line or every pixel.
Further, in K. Toda, et al., "A Modified-Binary-Coded Light-Emission Scheme for Suppressing Gray Scale Disturbances of Moving Images", ASIA DISPLAY'95, Oct. 17, 1995, pp. 947 to 948, a sub-field construction is proposed in which, for 256 gray levels, two sub-fields each weighted with a binary number corresponding to light intensity of 48 are arranged on each side of 6 sub-fields weighted with binary numbers corresponding to light intensity level of 1, 2, 4, 8, 16 and 32, respectively. Although the proposed sub-field arrangement substantially relaxes time variation in shift-up operation of bits, there are problems that it requires a number, as large as 10, of sub-fields for 256 gray levels and there is no suppression effect of pseudo contours of moving images with gray level change from light intensity of 31 to 32. This is because the proposed sub-field arrangement is based on the dispersion of light intensity from the upper sub-fields and an information which can be expressed by 10 bits is-not utilized effectively.
Among the conventional techniques mentioned hereinbefore, the method utilizing the optimization of the sequence of sub-fields is not sufficient for a high quality video image display since pseudo contours of moving images is not suppressed enough. Further, in order to obtain a sufficient suppression effect for the pseudo contours of moving images, it is necessary in the method in which the field time or display period is shortened or a number of sub-fields are divided to substantially shorten the scan period. This requirement can be satisfied by a plasma display having a display capacitance which is small enough to allow a sufficiently long scan period. However, a multi-level display of moving images is desired by a display having rather large display capacitance and it is difficult to drive such display with further substantial reduction of scan period.
That is, pseudo contours of moving images occur due to unevenness of shift time in shifting up by one gray level in the gray scale display method for displaying gray scale by combining a plurality of sub-fields light intensities of which are weighted by binary numbers. Conventionally, such unevenness of shift time is dispersed by employing special sub-field arrangement or division of upper sub-fields. However, there is no procedure taken to completely remove the time variation which is the cause of pseudo contours of moving images and, therefore, the effect of conventional method is limited. The time unevenness resides in the sub-field method using weighting light intensity with binary numbers and, unless this is solved, the problems inherent to the conventional methods can not be solved.